A number of different processes are currently in use to remove or scrub hydrogen sulfide (H2S) and carbon dioxide (CO2) from sour natural gas. Generally, these processes are also capable of removing carbonyl sulfide (COS), carbon disulfide (CS2) and mercaptans (RSH—where R is any radical). The processes include chemical and physical processes, batch processes, molecular sieve processes and membrane separation processes.
Caustic (sodium hydroxide) has traditionally been used to carry out a fine purification of the gas after the coarse treatment of a natural gas stream by a chemically or a physically regenerable solvent. Conventionally, a caustic solution countercurrently contacts with a sour gas mainly containing H2S and CO2 in a packed or trayed column. When CO2 is present, the sodium hydroxide solution will absorb CO2 as well as H2S without preference. This leads to high caustic consumption and the spent solution has to be neutralized by acid for disposal.
The chemical reactions involved in caustic scrubbing are as follows:NaOH+CO2→NaHCO3H2S+NaOH→NaSH+H2ONaSH+NaOH→Na2S+H2OH2S+2NaOH→Na2S+H2ORSH+NaOH→RSNa+H2OCO2+2NaOH→NaCO3+H2OCS2+2NaOH→2NaHS+CO2
The scrubbing liquors contain mixtures of sodium hydrosulfide, sodium bicarbonate, sodium carbonate and sodium carbamate in varying amounts. Depending on the composition of the gas which has been scrubbed and the operating conditions of the scrubber, there may be anywhere from 0 to 100% carbonate salts in the scrubbing liquors.
Various options exist to treat the liquors. One option is to fully oxidize the mixture to a mixture of sodium sulfate and sodium bicarbonate. Another option is to partially oxide the scrubbing liquors to a mixture of sodium thiosulfate and sodium bicarbonate. The mixture may then be acidified with sulfuric acid to decompose the bicarbonate to carbon dioxide. This forms a solution which is essentially pure sodium sulfate. The sodium sulfate formed in this way (or the sodium sulfate/sodium bicarbonate mixture) may then be treated in a bipolar cell or an electrochemical cell to regenerate a solution of sodium hydroxide. The regenerated sodium hydroxide may then be recycled to a column or mixer for example to be used for further scrubbing. U.S. Pat. No. 5,098,532 discloses a three compartment electrochemical cell that can be used to produce ammonium sulfate from sodium sulfate. Alternative electrochemical cells may also be used.
Another option for regeneration of caustic is countercurrent contacting of the liquid with 10% sodium hydroxide solution in a packed column. The caustic is regenerated in a stripping column by the addition of open steam or by steam internally generated by a column heating element. The condensate is returned to the stripping column to maintain caustic concentration.